High strength hot rolled steel sheet and method for producing the same

Abstract

A high strength hot rolled steel sheet has a matrix that has a ferrite phase with an area ratio of 95% or more with respect to an overall structure; and a structure where a fine carbide is dispersedly precipitated, the fine carbide containing Ti and V having an average particle size of less than 10 nm in the matrix, the fine carbide has a volume fraction of 0.0050 or more with respect to the overall structure, a proportion of a number of carbides with a particle size of 30 nm or more containing Ti is less than 10% with respect to a total number of carbides, the high strength hot rolled steel sheet has a tensile strength of 980 MPa or more.

1 .- 10 . (canceled) 
     
     
         11 . A high strength hot rolled steel sheet comprising:
 a chemical composition comprising:
 C: more than 0.05% to 0.13% or less; 
 Si: 0.3% or less; 
 Mn: 0.5% or more to 2.0% or less; 
 P: 0.025% or less; 
 S: 0.005% or less; 
 N: 0.0060% or less; 
 Al: 0.1% or less; 
 Ti: 0.07% or more to 0.18% or less; and 
 V: more than 0.13% to 0.30% or less on a mass percent basis, such that Ti and V contents satisfy formula (1),
   0.25<Ti+V≦0.45   (1)
 
 
 (where Ti and V are respective contents of the elements (by mass %)), wherein solid solution V: 0.05% or more to less than 0.15%, and balance comprising Fe and inevitable impurities, 
   a matrix that has a ferrite phase with an area ratio of 95% or more with respect to an overall structure;   a structure where a fine carbide is dispersedly precipitated, the fine carbide containing Ti and V having an average particle size of less than 10 nm in the matrix,   the fine carbide has a volume fraction of 0.0050 or more with respect to the overall structure, and   a proportion of a number of carbides with a particle size of 30 nm or more containing Ti is less than 10% with respect to a total number of carbides,   the high strength hot rolled steel sheet has a tensile strength of 980 MPa or more.   
     
     
         12 . The high strength hot rolled steel sheet according to  claim 11 , wherein the chemical composition further contains at least one selected from the group consisting of Nb and Mo by 1% or less in total on a mass percent basis. 
     
     
         13 . The high strength hot rolled steel sheet according to  claim 11 , wherein the chemical composition further contains at least one selected from the group consisting of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, B, Pb, Ta, and Sb by 1% or less in total on a mass percent basis. 
     
     
         14 . The high strength hot rolled steel sheet according to  claim 12 , wherein the chemical composition further contains at least one selected from the group consisting of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, B, Pb, Ta, and Sb by 1% or less in total on a mass percent basis. 
     
     
         15 . The high strength hot rolled steel sheet according to  claim 11 , further comprising a plating layer formed at a surface of the high strength hot rolled steel sheet. 
     
     
         16 . The high strength hot rolled steel sheet according to  claim 11 , wherein the high strength hot rolled steel sheet has a hole expansion ratio of 40% or more. 
     
     
         17 . The high strength hot rolled steel sheet according to  claim 16 , wherein the high strength hot rolled steel sheet has a ratio of limit bending radius of 0.9 or less. 
     
     
         18 . The high strength hot rolled steel sheet according to  claim 17 , wherein a difference in a tensile strength between a center position of a sheet width and a position at one-quarter width of the steel sheet is 15 MPa or less, a difference in a hole expansion ratio among the positions is 10% or less, and a difference in a ratio of limit bending radius among the positions is 0.15 or less. 
     
     
         19 . A method of producing a high strength hot rolled steel sheet comprising:
 preparing a steel material with a chemical composition according to  claim 11 ;   hot-rolling the steel material including rough rolling and finish rolling at a rolling temperature of 880° C. or more to form a hot-rolled steel sheet;   cooling the hot-rolled steel sheet at an average cooling rate of 10° C./sec. or more subsequent to completion of the finish rolling; and   coiling the hot-rolled steel sheet at 550° C. or more to less than 700° C.,   
     
     
         20 . The method according to  claim 19 , further comprising plating a surface of the hot-rolled steel sheet subsequent to the coiling. 
     
     
         21 . The high strength hot rolled steel sheet according to  claim 12 , further comprising a plating layer formed at a surface of the high strength hot rolled steel sheet. 
     
     
         22 . The high strength hot rolled steel sheet according to  claim 13 , further comprising a plating layer formed at a surface of the high strength hot rolled steel sheet. 
     
     
         23 . The high strength hot rolled steel sheet according to  claim 14 , further comprising a plating layer formed at a surface of the high strength hot rolled steel sheet. 
     
     
         24 . The high strength hot rolled steel sheet according to  claim 12 , wherein the high strength hot rolled steel sheet has a hole expansion ratio of 40% or more. 
     
     
         25 . The high strength hot rolled steel sheet according to  claim 13 , wherein the high strength hot rolled steel sheet has a hole expansion ratio of 40% or more. 
     
     
         26 . The high strength hot rolled steel sheet according to  claim 14 , wherein the high strength hot rolled steel sheet has a hole expansion ratio of 40% or more. 
     
     
         27 . A method of producing a high strength hot rolled steel sheet comprising:
 preparing a steel material with a chemical composition according to  claim 12 ;   hot-rolling the steel material including rough rolling and finish rolling at a rolling temperature of 880° C. or more to form a hot-rolled steel sheet;   cooling the hot-rolled steel sheet at an average cooling rate of 10° C./sec. or more subsequent to completion of the finish rolling; and   coiling the hot-rolled steel sheet at 550° C. or more to less than 700° C.   
     
     
         28 . A method of producing a high strength hot rolled steel sheet comprising:
 preparing a steel material with a chemical composition according to  claim 13 ;   hot-rolling the steel material including rough rolling and finish rolling at a rolling temperature of 880° C. or more to form a hot-rolled steel sheet;   cooling the hot-rolled steel sheet at an average cooling rate of 10° C./sec. or more subsequent to completion of the finish rolling; and   coiling the hot-rolled steel sheet at 550° C. or more to less than 700° C.   
     
     
         29 . A method of producing a high strength hot rolled steel sheet comprising:
 preparing a steel material with a chemical composition according to  claim 14 ;   hot-rolling the steel material including rough rolling and finish rolling at a rolling temperature of 880° C. or more to form a hot-rolled steel sheet;   cooling the hot-rolled steel sheet at an average cooling rate of 10° C./sec. or more subsequent to completion of the finish rolling; and   coiling the hot-rolled steel sheet at 550° C. or more to less than 700° C.